1. Field of the Invention
The subject disclosure relates to implantable medical leads in general, and more particularly to an implantable bipolar cardiac lead with an anodic electrode assembly having dual support hulls.
2. Background of the Related Art
Abnormal heart beats, knows as arrhythmias, can include a heart beating too rapidly, too slowly, or irregularly. These conditions can be treated by applying electrical energy to the heart. It is, however, important that this treatment be implemented shortly after the onset of an episode of arrhythmia, as episodes not treated within minutes can be fatal.
One method for applying electrical energy to the heart is to implant a pulse generating device in the body that senses arrhythmia and administers an appropriate amount of electrical energy to the heart tissue. The pulse generator is connected to the heart using one or more cardiac leads, each incorporating one or more electrodes that directly engage the heart. Because these leads and electrodes are a necessary part of a life-sustaining process, it is essential that they operate reliably for long periods of time. Further, because these devices are implanted as part of an invasive procedure, and then remain permanently inside the body, it is preferred that they be as small as possible.
There are two major types of cardiac leads in use. Unipolar leads are those that provide only one electrode (the cathode) for electrical connection to the heart. Electrical impulses travel through the lead and electrode to the heart and then return through the person's body to the generating device (which serves as the anode) to complete the circuit. Bipolar leads provide two electrodes for connecting to the heart, a cathode and an anode. Electrical impulses travel through the cathode, continue a short distance through the heart, and then reach the anode which completes the circuit. Bipolar leads are generally preferred over unipolar due to their greater insensitivity to extraneous electromagnetic interference. However, bipolar leads typically have the disadvantage of being larger than unipolar leads.
Several alternative configurations for bipolar cardiac leads have been developed previously. For example, see U.S. Pat. No. 4,590,950 of Iwaszkiewicz; U.S. Pat. No. 4,628,943 of Miller; French patent application No. 81 19037 of Sandstrom et al.; and German patent application No. 37 18 324 A1 of Hirschberg. Despite such advances in the field, many teachings lack a positive electrical connection between the anodic electrodes contacting the heart and the conductors within the leads that supply electrical impulses from the generating device. This lack of positive connection leads to reduced mechanical reliability of the lead.
One invention which does not suffer from the above drawback is presented in German patent application No. 30 43 189 C3 to Osypka (“the '189 application”). The '189 application discloses an electrode 1 in which several conductors 5,6 arranged in a multiple helix extending through a hose 7. Some of the conductors connect to a pole 4 at the end of the electrode 1, while others pass through the hose 7 and are affixed to a pole 3 located around the hose 7. This configuration provides a positive electrical connection between the conductors 5,6 and the poles 3,4. Further, the connections remain completely shielded from the body's environment by the pole 3 and hose 7. However, the solid structure of the pole 3 disclosed in the '189 application makes the process of attaching the conductor 5 quite difficult.
U.S. Pat. No. 5,251,643 to Osypka (“the '643 patent”) discloses a cardiac pacemaker lead 1 having a proximal end 1a and a distal end 1b, the distal end 1b having a first pacing electrode 15. A first helically wound, wire-like conductor 4 is connected to the first pacing electrode 15 and extends toward the proximal end 1a. A second helically wound wire-like conductor 3 similarly extends toward the proximal end 1a. A first tubular, sheath-like insulator 9 surrounds the convolutions of the conductor 3 and extends all the way to the electrode 15. A second tubular, sheath-like insulator 8 is disposed between the conductors 3,4 and also extends all the way to the electrode 15. A tubular fourth conductor 7 is confined between the convolutions of the conductor 3 and the insulator 8 and its distal end 7a is electrically connected to the conductor 3 at its last convolution 3d using a solder or weld joint 3c. A sleeve-like second pacing electrode 5 is fit on an adjacent portion of the insulator 9 at a location 2 between the electrode 15 and the proximal end 1a. An elongated, band-shaped third conductor 6 has a second portion 6a attached to the conductor 7 by solder or weld joints 13a. An intermediate portion 6c of the conductor 6 extends between and contacts at least two neighboring convolutions of the conductor 3. A first portion 6b of the conductor 6 is adjacent to the internal surface of the electrode 5 and is connected thereto by a solder or weld joint 13.
The lead 1 of the '643 patent allows for positive connections between all of components in electrical contact. All connections are internal to the lead, thereby shielding the connections from the body's environment. Both of these factors contribute to the lead's increased reliability. However, the necessity for the third conductor 6 to make the connection between the second conductor 3 and the second electrode 5 increases the difficulty of assembly. Also, the use of coiled wires of different radius 3,4 increases the size of the lead and the fabrication process complexity.
Therefore, there is a need in the art for an elongated electrical lead capable of providing multiple, independent electrical connections while maintaining a small size and a simple assembly process relative to the prior art. Further, it is desired that the lead's internal electrical connections be substantial and be protected from the ambient environment in order to allow reliable operation.